These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fanon Discontinuity: Many fans like to assume the last episode of the first season, "The Circle," is actually set at the end of the second season, allowing the Marshalls to get home, and ignoring the third season entirely.
I Am Not Shazam: Despite what the theme song says, "Marshall" is the family's last name; the dad's name is Rick.
Seasonal Rot: The third season got left out of the recent marathons on the Sci-Fi Channel and Chiller. With good reason.
Special Effect Failure: Claymuppet dinosaurs, blindingly mismatched green-screen transfers, yetis wrapped in shag carpeting, and a unicorn with a chinstrap, among many, many other examples. The Dopey puppet was a particularly bad example.
What The Hell, Casting Agency?: Future NBA Star Bill Laimbeer playing a Sleestak in the series. (Note that this was long before he made it to the NBA; he was only seventeen at the time.)
There are very few actors tall enough for Sleestak. There are many basketball players tall enough.
Rick and Holly becoming a couple, considering she was his daughter in the show. Particularly squicky when he gives her a compliment at one point that makes no sense except as something a father would say to a daughter.
Then again, at times Will and Holly's relationship in the original series had some incestuous overtones. This Troper recalls in ep. 2 how, when trapped by Sleestaks and hanging in a net, Will touches his sister's cheek, looks in her eyes like a lover and tell her how brave she is. I was all, "Oh, hell, no".